The laser is a device for the generation of coherent, nearly single-wavelength and single-frequency, highly directional electromagnetic radiation emitted somewhere in the range from submillimeter through ultraviolet and x-ray wavelengths. The word laser is an acronym for the most significant feature of laser action: light amplification by stimulated emission of radiation.
There are many different kinds of lasers, but they all share a crucial element: each contains material capable of amplifying radiation. This material is called the gain medium, because radiation gains energy passing through it. The physical principle responsible for this amplification is called stimulated emission. It was widely recognized that the laser would represent a scientific and technological step of the greatest magnitude, even before T. H. Maiman constructed the first one in 1960. Laser construction generally requires three components for its operation: (1) an active gain medium with energy levels that can be selectively populated; (2) a pumping process to produce population inversion between some of these energy levels; and usually (3) a resonant electromagnetic cavity structure containing the active gain medium, which serves to store the emitted radiation and provide feedback to maintain the coherence of the electromagnetic field.
Many lasers have the capability to emit light over a tunable wavelength range. For a laser to be tunable in wavelength it must possess a laser gain medium whose spectral gain bandwidth is tunable, with temperature or by some other means. Alternatively, the spectral gain of the gain medium can be broad, and an additional wavelength-dependent loss element is added to the resonator to tune the laser emission to different wavelengths within the spectral gain curve.
The lasers of the present invention use a new crystal material, trivalent ytterbium-doped yttrium calcium oxyborate crystals and are referred to herein as Yb.sup.3+ :YCa.sub.4 O(BO.sub.3).sub.3 or Yb:YCOB for easier reference.
A Patent Corporation Treaty (PCT) application numbered WO 96/26464 reports the growth of calcium gadolinium oxyborate, GdCOB, as the first element of a new family of borate crystals, which includes YCOB. However, WO 96/26464 does not disclose or suggest a tunable laser device comprising Yb:YCOB.
In the prior art, there are no disclosures of Yb:YCOB being used as the active gain medium. Further, there are no teachings supporting the use of Yb:YCOB to generate tunable, self-frequency doubled, coherent, visible laser light or ultrashort infrared radiation pulses.
Trivalent ytterbium-doped crystalline laser systems producing optical radiation are reported. U.S. Pat. No. 3,462,707 discloses Yb and Nd doped borate glass host for a non-radiative transfer of energy between Nd ions and Yb ions; there is no mention of frequency doubling. U.S. Pat. No. 5,123,026 disclosed that a Yb-doped host crystal from the garnet family worked as a laser with a separate frequency doubling crystal located within the resonant cavity. Other Yb-doped host material are described in U.S. Pat. Nos. 5,280,492 and 5,381,428; the crystals from the classes of oxides, fluorides, fluoroapatite or glass. Frequency doubling is accomplished by a separate crystal placed in the laser cavity. Tuning is accomplished in U.S. Pat No. 5,381,428 with a birefringent tuning plate, a grating, or a prism also placed within the laser cavity. U.S. Pat. No. 5,677,921 discloses a new class of laser crystals formed from Yb-doped borate fluoride host crystals; these crystals were found to be self-frequency doubling.
More recently, the approach to generating high power, visible laser light has been to use nonlinear optical crystals to convert near-infrared radiation to the visible portion of the spectrum via second harmonic generation (SHG) (sometimes termed frequency doubling and used interchangeably, herein). This process generates a harmonic wavelength which is one-half of the fundamental wavelength. Since the SHG conversion efficiency is a function of the fundamental laser beam intensity, the nonlinear crystal is often placed inside the cavity of a low power continuous wave laser to benefit from the high intracavity fundamental beam intensity.
Thus, in the search for smaller, less expensive, more powerful, multifunctional lasers, the discovery of a new class of laser hosts, the oxyborates, makes possible the combination of linear and nonlinear optical properties in a single active medium. More particularly, the ytterbium-doped oxyborate crystal (Yb:YCOB) of the present invention generates an infrared fundamental light over a relatively wide bandwidth, from approximately 980 nanometers (nm) to approximately 1100 nm. This approximately 100 nm range is a large bandwidth which could support the generation of pulsed infrared light or when self-frequency doubled provides a compact, efficient, source of tunable, visible blue or green laser light with wavelengths of approximately 490 nm to approximately 550 nm.